1. Field of the Invention
This invention relates to a pressurizing control method and a pressurizing control system for low-pressure casting in which molten metal is lifted into a casting mold by a pressure applied to the molten metal surface.
2. Description of the Prior Art
When casting an aluminum cylinder head of an automotive engine or the like, there has been employed so-called low-pressure casting in which molten metal in a crucible is lifted into a cavity of a casting mold through a stalk under a pressure applied to the surface of the molten metal by a pressurizing means.
Generally in a low-pressure casting apparatus, it is preferred to rapidly pressurize the molten metal surface until the molten metal reaches the gate of the casting mold in order to prevent a drop in the temperature of the molten metal, and it is required to pressurize the molten metal surface under a suitable pressure determined according to the shape of the product and the like from the time the molten metal begins to be supplied to the cavity to the time the molten metal fills the cavity. Further after the molten metal fills the cavity, it is desired to pressurize the molten metal surface under a high pressure in order to solidify the molten metal under a good condition. For this purpose, in a low-pressure casting apparatus, the pressure applied to the molten metal surface is controlled in a preset pressurizing pattern.
In order to obtain a dense cast product free from shrinkage void, the pressure applied to the molten metal surface is to be constantly proper in a filled state where the cavity is filled with the molten metal. As a system for controlling the pressure to be optimal in a filled state, there has been known a system disclosed in Japanese Unexamined Patent Publication No. 2(1990)-251355.
In the low-pressure casting apparatus, a filling sensor which detects that the cavity has been filled with molten metal and outputs a filling signal and a timer which outputs an elapse signal when a predetermined time lapses after starting of molten metal supply to the cavity are provided, and the pressurizing pattern is changed on the basis of the filling signal when the filling signal is output before the predetermined lapses and on the basis of the elapse signal when the filling signal is not output before the predetermined lapses.
Accordingly, the pressure applied to the molten metal surface in the filled state can be constantly controlled to be optimal and a dense product structure free from void can be obtained.
However, the low-pressure casting apparatus is disadvantageous in that the pressure applied to the molten metal surface is controlled in a preset pattern before the cavity is filled with the molten metal and accordingly, only a predetermined pressure is applied to the molten metal surface even if the molten metal in the crucible is reduced and the molten metal level lowers, which makes it difficult to supply the molten metal at a desired rate. When the predetermined lapses and the elapse signal is output, the pressurizing pattern is changed even if the cavity is not filled with the molten metal yet, whereby casting defect is generated and the cast product cannot be of a desired shape.